The present invention relates, in general, to an exhaust system for a motor vehicle.
Internal combustion engines, in particular Otto engines with direct fuel injection, typically run in lean-combustion operation and have understoichiometric operating points in order to realize a lowest possible fuel consumption. As a consequence, current catalytic converters, in particular three-way catalytic converters, are not capable to completely convert harmful components hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) into non-toxic compounds such as carbon dioxide (CO2), nitrogen (N2) or water (H2O). The reason for the incomplete conversion of the harmful components is low exhaust temperatures in the test cycle, higher HC-emissions during lean-combustion operation of the Otto engines, and in particular the need for NOx-reduction in the oxygen-rich exhaust gas.
In order to achieve NOx efficiencies of above 90% at a temperature range between 300xc2x0 C. and 500xc2x0 C., the provision of a particular exhaust aftertreatment is proposed. One approach of an exhaust aftertreatment involves the arrangement of a three-way catalytic converter near the engine and a NOx-adsorber underneath the bottom of the motor vehicle. Such an arrangement must, however, reconcile two contradicting requirements. On the one hand, the three-way catalytic converter should be raised in shortest possible time (light off time) to an operating temperature of  greater than 350xc2x0 C. On the other hand, the exhaust temperature should not exceed 500xc2x0 C. to prevent adverse effect on the maximum temperature resistance of the NOx-adsorber.
To date, the first requirement has been realized through use of air-gap insulated exhaust lines of double-walled configuration, comprised of a thin-walled inner tube with slight heat storage capacity, resulting in a low heat withdrawal from the exhaust and thus in a rapid heating of the catalytic converter, and an outer tube, which realizes the tightness to the outside and provides also a load-carrying function.
The second requirement, however, has been difficult to meet satisfactorily heretofore because of the presence of temperatures of well above 500xc2x0 C., in particular in the high-load range or high-speed range, upstream of the NOx-adsorber. As a consequence, it has been proposed to provide fairly long exhaust lines to thereby cool the exhausts on their way to the NOx-adsorbers. Still, the temperature stress remains very high so that the service life of currently available NOx-adsorbers is insufficient.
Attempts have been undertaken to employ engine-internal measures, such as cooling by means of fuel, i.e. enrichment of the fuel/air mixture, in order to lower the temperature stress. Such engine-internal measures significantly lead, however, to a deterioration of the overall efficiency.
It would therefore be desirable and advantageous to provide an improved exhaust system which obviates prior art shortcomings and which can be configured to include a three-way catalytic converter and a NOx-adsorber while still resulting in a lower fuel consumption and less emission into the environment as well as is able to prolong the service life of the NOx-adsorber.
According to one aspect of the present invention, an exhaust system used for a motor vehicle and extending between an exhaust-generating internal combustion engine and an exhaust pipe for release of exhaust into the surrounding environment, includes a three-way catalytic converter disposed in proximity of the combustion engine; a NOx-adsorber disposed upstream of the exhaust pipe, an exhaust line extending between the catalytic converter and the NOx-adsorber for conducting exhaust, wherein the exhaust line includes an inner tube, an outer tube in spaced-apart surrounding relationship to the inner tube to define a space therebetween, and a separation tube disposed in the space between the inner tube and the outer tube, to thereby define between the inner tube and the separation tube an inner annular gap for conducting exhaust, and between the outer tube and the separation tube an outer annular gap for conducting a coolant; and a shut-off device for regulating a flow of exhaust through the inner tube.
The present invention resolves prior art problems by applying the double-wall technique with inner tube and separation tube to form the insulating inner annular gap for conduction of the exhaust at slight heat loss so as to realize a rapid heating of the three-way catalytic converter, in combination with the formation of a further outer annular gap between the separation tube and the outer tube for conduction of coolant. Thus, after activation of the shut-off device to bar a flow through the inner tube, exhaust is able to flow only through the inner annular gap for intimate contact with the coolant to thereby realize a cooling of the exhaust. As a result, exhaust admitted to the NOx-adsorber is effectively cooled down to a temperature that is appropriate for the NOx-adsorber.
The invention thus reconciles the seemingly contradictory requirements of implementing a rapid heating of the three-way catalytic converter and cooling of exhaust after this heating for processing by the NOx-adsorber. Fuel consumption is more efficient while reducing emission from the exhaust system. Since the NOx-adsorber is no longer exposed to excessive temperatures, its service life is significantly prolonged. As a consequence of the clear functional separation, different components of the exhaust system may be manufactured of specifically suitable materials so that the service life of those components that are used for exhaust conduction is prolonged as well. Moreover, weight of the exhaust system may be reduced.
According to another feature of the present invention, the inner tube has an entry zone for the exhaust, with the entry zone being fluidly connected to the inner annular gap. This may be realized by providing the wall of the inner tube with suitable transfer apertures.
According to another feature of the present invention, the shut-off device may be disposed in the entry zone of the inner tube. Suitably, the shut-off device is a butterfly valve. Operation of the shut-off device may be realized in dependence on a temperature level of the exhaust in the exhaust line upstream of the NOx-adsorber.
According to another feature of the present invention, the inner tube has an engine-distal end which is constructed to allow relative mobility or adjustability of the inner tube. In this way, different expansions of various components of the exhaust system, caused by different temperatures, can be compensated. The mobility of the inner tube may be realized by providing the engine-distal end of the inner tube with a sliding seat. Suitably, the engine-proximal end of the inner tube may be welded to the catalytic converter.
According to another feature of the present invention, there may be provided a cooling unit, which is fluidly connected to the outer annular gap for re-cooling the coolant after flowing in counterflow direction to the exhaust. The cooling unit may include a heat exchanger and a circulation pump for urging the coolant to flow in counterflow direction to the exhaust. Suitably, the heat exchanger may be an air/coolant heat exchanger.
The tubes are suitably formed by means of internal high-pressure technique so that cross sections other than round cross section, as well as punchings for passages can be realized at smaller manufacturing tolerances.